1. Field of the Invention
This invention relates to a light emission type display apparatus, for example, using light emitting diode arranged in a matrix form.
2. Description of the Related Art
Color image display apparatuses using an organic light emitting diode (which will be referred to as an OLED hereinafter) have recently attracted much attention. Among others, an active matrix type OLED display which drives many OLED elements arranged in a matrix form by using respective thin film transistors appears promising as a mode which is suitable for realization of a high definition and an increase in size.
As a pixel circuit of the active matrix type OLED display, there has been very commonly used a circuit which comprises two transistors and one capacitive element that is a sampling transistor which samples an analog image signal, a memory capacitance which stores an image signal and a drive transistor which controls a current which is supplied to an OLED in accordance with an image signal voltage stored in the memory capacitance. Jpn. Pat. Appln. KOKAI Publication No. 2002-156923 discloses an example of such a circuit configuration.
In the above-described prior art, an intensity of the current supplied to the OLED is controlled by applying an analog voltage held in the memory capacitance between a gate and a source of the drive transistor and driving the drive transistor in a saturation region, i.e., a region in which a voltage between a drain and the source becomes larger than a voltage between the gate and the source. By operating the drive transistor in the saturation region in this manner, a drop in a voltage between terminals of the OLED element varies due to a change in a current value, and a drive current value can be maintained at a fixed value even if the voltage between the source and the drain of the drive transistor varies. Therefore, the drive transistor must have excellent saturation characteristics. Here, the excellent saturation characteristics means that the voltage range in which the drain current can be fixed is wide, regardless of the voltage between the source and the drain.
Because of such a demand relative to the performance of the drive transistor, there is usually used a long-channel transistor whose channel length is very long, e.g., 10 μm to 20 μm as the drive transistor. That is because it is hard to obtain saturation characteristics that the drain current has a fixed value with respect to the voltage between the source and the drain, due to a channel length modulation effect or a parasitic bipolar effect when a channel length is short in a regular MOS type transistor. Further, another reason is that a breakdown voltage between the source and the drain is reduced as a result of an increase in an electric field in the vicinity of a drain junction due to a reduction in a channel length.
However, when long-channel transistor whose channel length is 10 μm to 20 μm is formed in a pixel, since an area occupied by the transistor is increased, an area in which an OLED element can be formed is decreased, thereby reducing an open area ratio. Therefore, a current density to be supplied to the OLED element must be increased in order to assure a fixed brightness, which results in a problem that a deterioration in the OLED elements is accelerated and a lifetime of a product is shortened.